Having grown up playing Gauntlet in the arcades and Castlevania at home, I've always liked the idea of weak but plentiful ghosts that the heroes can strike down with a couple of blows, and without the need for magical weapons. Basically, immaterial skeleton/zombie level undead baddies.
But of course the ghosts that appear in the AD&D DMG and Companion Set/RC are not what I'm looking for.
So here's a monster write-up for some weaker cannon-fodder ghosts.
Armor Class: 5 (15)
Hit Dice: 1* to 3*
Move: Fly 90 (30)
Attacks: 1 touch
No. Appearing: 3-12 (3-18)
Save As: Fighter 1-3
Treasure Type: U
XP: 13, 25, or 50
Restless Spirits are animated remnants of souls that are constrained to the mortal realm. They have only the faintest of intelligence, and seek out living beings to feed upon. They are immaterial, and may pass through solid matter such as walls, floors or ceilings to attack. As undead creatures, they are immune to sleep, charm and hold magics. Restless Dead are turned as follows: 1HD as skeletons, 2HD as zombies, 3HD as ghouls.